1. Field of the Invention
The present invention relates to a gear change control system of an automatic transmission which is provided on a drive train of a vehicle.
2. Description of the Related Art
There is a mechanical automatic transmission which does not use a torque converter as a transmission provided on a drive train of a vehicle.
This mechanical automatic transmission obviates the necessity of a torque converter by implementing actuation of a gear change control (selection and implementation of a gear shift) and engagement and disengagement of a clutch which are to be performed in a manual transmission by an actuator. For example, as is shown in FIGS. 8A and 8B, a gear change control system of an automatic transmission includes a shift shaft 100 which can move in a shifting direction sf and a selecting direction se, shift lugs 120 which are formed, respectively, at one parts on a plurality of shift rails 110 which are arranged in the selecting direction se, in such a manner as to project radially outwards therefrom and shift forks 131, 132, 133 which are connected integrally to another parts of the shift rails 110, respectively.
Furthermore, a control finger (an arm portion) 140 is provided on the shift shaft 100 in such a manner as to project radially therefrom, and a pair of claw portions 121 are formed on each shift lug 120 in such a manner as to be spaced apart from each other in the shifting direction sf.
In the mechanical automatic transmission configured as has been described above, by moving the shift shaft 100 by an actuator (not shown), the pair of claw portions 121 of one of the shift rails 110 are selectively moved in one or the other of the shifting directions sf by the control finger 140, so that the shift fork 131 which is linked with the shift rail 110 is made to shift a gear which faces oppositely the shift fork 131.
Incidentally, a dual-clutch automatic transmission has been developed in which two clutches are incorporated between an automatic transmission like the one described above and an engine which is a power source. This automatic transmission includes first and second main shafts, and one and the other of the main shafts change the speed of a rotational force transmitted thereto from the associated clutches for transmission to countershafts they face oppositely, the speed-changed rotation being then transmitted from each of the countershafts to an output gear side of the transmission. In the dual-clutch automatic transmission configured described above, in making a gear shift, a state in which one gear is engaged with one of the clutches via the first main shaft is switched to a state in which a target gear is engaged with the other clutch via the second main shaft, and as this occurs, a neutral state can be eliminated during the gear change by releasing gradually the engagement of the one of the clutches while engaging gradually the other clutch on the target gear side, thereby making it possible to realize a smooth gear change without any interruption of power flow during the gear change.
In a gear change control system adopted in the transmission described above, for example, as is shown in FIG. 9A, it assumes that a gear change has been made from a fourth gear which is a gear currently engaged to a target gear, for example, a first gear while maintaining the state in which the current gear is engaged. Immediately thereafter, as is shown in FIG. 9B, while the synchronization in rotational speed of the target gear with the engine speed is in progress, a gear shift withdrawal from the previous gear (here, the fourth gear) is implemented. To describe this by reference to a locus indicated by a chain double-dashed line in the figure, in the gear shift withdrawal from the previous gear position, the following steps need to be implemented quickly: firstly, a step (1) in which the control finger is offset from the target gear (the first gear) position in the selecting direction, a shift step (2) in which the control finger returns to a neutral line N, a selecting step (3) in which the control finger moves towards the gear (the fourth gear), a shift step (4) in which the control finger moves towards the gear (the fourth gear), a selecting step (5) in which the control finger reaches the gear (the fourth gear) position, and a gear shift withdrawal step (6) in which the control finger withdraws the gear shift made to the gear (the fourth gear) to a neutral position (indicated by the chain double-dashed line) thereof.
In addition, JP-A-2001-304411 proposes an automatic transmission in which a pair of claw portions are provided on each shift lug in such a manner as to be spaced apart widely from each other in a shifting direction, so that a control finger can enter between the pair of claw portions on the shift lug of a target gear which is in a neutral state only by being moved in a selecting direction from between the pair of claw portions of the shift lug which is in the shifted state, so as to simplify the movement of a shift member.
In this way, in the dual-clutch automatic transmission, since the gear shift to the target gear and the gear shift withdrawal from the currently engaged gear need to be performed in that order during gear change, the movement of the control finger 140 gets complex, resulting in a cause for extending the gear change time, and an improvement in this area has been longed for.
Furthermore, in the related art disclosed in JP-A-2001-304411, in order for the arm portion (the control finger) of the shift member to enter between the pair of claw portions when the arm portion is moved directly in the selecting direction, the pairs of claw portions each have to be disposed in such a manner as to be spaced apart relatively widely from each other. In this case, as is shown in FIG. 7, in the event that an interval L1 between the pair of claw portions 121 is increased, the arm portion 140 of the shift member pushes on the claw portion 121 in such a state that the arm portion 140 is largely inclined in the shifting direction. As this occurs, it assumes that a force acting on the claw portion 121 by a rotational torque T of the control shaft 100 is a tangential force F, a distance from a contact point a between the arm portion 140 of the shift member and the claw portion 121 to an axial center C of the control shaft 100 is a distance L2, and an inclination angle of a line connecting the contact point a with the axial center C towards the shifting direction is an angle α, a component force of the tangential force F in the shifting direction, that is, a component force P1 which constitutes a force for moving the claw portion 121 in the shifting direction is obtained by the following expression (1).P1=F×COS α=(T/L2)×COS α  (1)
In the expression (1) above, it is found that with the distance L2 being substantially constant irrespective of the angle α, the component force P1 decreases as the angle α increases within a range of 0 to 90 degrees. In addition, it is found that the distance L2 increases and the component force P1 decreases further as the angle α increases within a range of 0 to 90 degrees. Consequently, in the event that the interval L1 between the pair of claw portions 121 is increased as with JP-A-2001-304411, the force P1 which pushes on the claw portion 121 in the shifting direction decreases, and it becomes difficult to move the shift lug which is linked with the claw portion 121 in the shifting direction with good efficiency.